Since the metered dose pressurised inhaler was introduced in the mid-1950's, inhalation has become a widely used route for delivering bronchodilator drugs and steroids to the airways of asthmatic patients. More recently, inhalation from a pressurised inhaler has been a route selected for administration of other drugs that are not primarily concerned with treatment of a bronchial malady.
A pressurised metered dose inhaler generally comprises an aerosol container equipped with a metered dose dispensing valve (which is generally herein referred to as a canister) and an actuator. The actuator generally comprises a nozzle block to retain the canister (typically the valve stem of the metering valve) and a user-port such as a mouthpiece, as well as an optional, but typically used, cylindrical housing for housing the aerosol container. The aerosol container contains a pressurised aerosol formulation that generally comprises a liquefied propellant and a medicament and, if desired and/or needed, one or more excipients such as a surfactant or a solvent. The medicament may be in the form of a dispersion or in solution in the aerosol formulation. Metered dose dispensing valves generally comprise a valve stem that is moved inwardly with respect to the container to dispense a metered dose of aerosol formulation. The canister is inserted into the actuator with the valve stem engaging the nozzle block of the actuator. In use, a patient places their lips round the mouthpiece and presses the base of the aerosol container causing the container to move relative to the valve stem to fire a dose of medicament through the mouthpiece.
One of the disadvantages arising from the use of such known inhalers is that the patient cannot readily determine the amount of medicament in the container at any given time. In an extreme case this could mean that a patient in need of a dose of medicament might find that the inhaler will not dispense a dose because its contents have already been exhausted.
There have been many proposals for dose counters for use with metered dose inhalers, in particular pressurised metered dose inhalers, including e.g. dose counters described in DE 10 2006 049 614 (RPC Formatec GmbH), WO 2000/059806 (Allsop et al.), WO 1998/056444 (Rand et al.), WO 2005/060535 (Purkins et al.), WO 2007/124406 (Stuart et al.) and WO 1993/024167 (Holroyd). Some are mounted onto the base of the aerosol container (e.g. WO 1993/024167), some are mounted onto the ferrule of the aerosol container (e.g. WO 1998/056444) and others are retained within the interior of the actuator beneath the aerosol container in the space near and or around the nozzle block (e.g. WO 2007/124406 and WO 2005/060535).